The present invention relates to an electric power steering control device for easing steering effort of the driver in a vehicle wherein motor power is directly applied to the steering system.
Electric power steering apparatus includes an electric motor in a steering system, and assists steering effort of the driver by way of applying power from the electric motor (hereinafter referred to as assist steering force) under the control of a control device. During the steering operation of the driver, front wheels are turned by a steering shaft to change the direction of the vehicle. For example, Japanese Laid-open Patent Application No.8-67266 discloses a construction for easing steering effort of the driver, wherein an electric motor which generates assist steering force is arranged coaxially with a rack shaft and through a ball screw mechanism rotational driving force of the electric motor is converted into reciprocal movement of the rack shaft. The control device controls the output power of the electric motor with a steering torque input as a main input and with an electric motor driving signal output as an output. The electric motor driving signal indicates a voltage difference to be applied between terminals of the electric motor. The control device includes an H-bridge so that forward and reverse controls can be achieved by switching current flow directions of the H-bridge. The electric motor driving signal is controlled to be a desired value with pulse width modulation control (hereinafter referred to as PWM control) relative to the switch of the H-bridge. Accordingly, the control device controls the output power of the electric motor to assist and supply thrust of the rack shaft within the steering gear box.
However, in the conventional electric power steering apparatus, the control calculation method of the control device is stored in a ROM (see, for example, [0036] to [0046] of the aforementioned publication). In other words, the control device is manufactured as a dedicated control device for a specific steering gear box intended for the control. For this reason, if the control specification is varied in accordance with characteristics of the intended steering gear box and conditions of the vehicle into which the steering gear box is assembled, a large number of control devices have to be prepared for the respective specifications. This will cause the following drawbacks.
(1) In the case where specifications of steering gear boxes or vehicles are different, it is necessary to manufacture control devices each having own specification.
(2) In the case where electric power steering apparatus with different steering feelings are required, it is necessary to manufacture control devices each having special map data.
(3) In the case where there are differences in engine rotational speed according to specifications of engines of the intended vehicles or there are differences in specifications of vehicle speed signals according to the intended vehicles, it is necessary to manufacture a dedicated control device for each intended vehicle.
In view of the above, the present invention seeks to provide an electric power steering control device in which the control calculation method of the electric power steering control device can be readily changed.